The coexistence of two major systems of measurement; namely, the English system and the metric system, is of some concern to persons confronted with the necessity of converting measurements made in one system of units to corresponding measurements in the other system of units. This is due, to a large extent, to the fact that for each conversion, there is a different conversion factor, so that the multiplicity of conversion factors is difficult to retain by memory and the conversion factor is often inconvenient to obtain.
The need for simple mechanisms which will convert measurements made in an English system to measurements made in the metric system has increased in the last few years in the United States primarily because of the proposed change from the English system to the metric system in this country. One area of especial concern is the changeover required when automobile speeds are registered in kilometers per hour, as opposed to the present day miles per hour, since most automobile speedometers are indexed in miles per hour. There is therefore a need for a convenient mechanism which can be associated with those speedometers indexed in miles per hour to convert same over to kilometers per hour without altering the speedometer itself.
In the conversion between miles per hour and kilometers per hour, several solutions to the problem have been proposed: One has been to open the speedometer dial and replace the index plate with a plate calibrated in kilometers per hour, or to place a transparent "paste-over" over the dial so that the needle position could be read through the transparent "paste-over" with reference to the modified markings.
Another solution to the problem is proposed in U.S. Pat. No. 3,446,178 wherein a special speedometer is constructed with two dials on opposite sides of a disc, and this disc may be rotated between a "miles per hour" and a "kilometers per hour" position externally of the speedometer. By having a circular dial, however, the design of a speedometer is limited to a circular configuration, and there would be the requirement that every automobile be equipped with the speedometer described since it requires replacement of the entire unit. While U.S. Pat. No. 3,446,178 does provide a solution to the above-described problem, its structure does not lend itself to the attachment to existing speedometers on a wide variety of makes and models of automobiles.
In standard speedometers, the position of the speedometer needle is dependent upon the rate of revolution of the input speedometer cable. The reason is that the speedometer cable is connected to the speedometer, and, as it turns under the driving force of the vehicle, such as the transmission or a wheel, the speedometer cable rotates a magnetic disc within a metal cup which is attached to the dial pointer or needle of the speedometer. This metal cup is retained by a return spring, but the rotation of the magnetic disc induces eddy currents creating a torque proportional to the rate of disc rotation which in turn causes the dial pointer to move across the dial face. For a given rate of revolution, the eddy current torque will reach an equilibium with the force exerted by the return spring, and the needle will register the speed of the vehicle. With this system, it therefore becomes possible to convert the speedometer reading from miles per hour to kilometers per hour by altering the rate of revolution of the speedometer cable by means of an appropriate conversion factor.